Method of obtaining strips and sheets of zinc and zinc alloys

ABSTRACT

A METHOD FOR PREPARING SHEETS OR STRIPS OF ZINC OR ALLOYS OF ZINC BY DIRECT ROLLING OF METAL PARTICLES IS DISCLOSED. THE SIZE OF THE PARTICLES IS PREFERABLY ABOVE A CERTAIN MINIMUM, THAT IS, LARGER THAN ABOUT 0.2 MM. IN THE SMALLEST DIMENSION, AND THE TEMPERATURE AND PRESSURE OF THE ROLLING ARE ALSO ABOVE CERTAIN MINIMUMS, THAT IS, SUFFICIENT TO EFFECT IN A SINGLE ROLLING PASS A PERCENT REDUCTION OF THE METAL OR ALLOY OF AT LEAST ABOUT 80% FROM THE ORIGINAL THICKNESS OF THE COMPACTED PARTICLES. A METHOD FOR THE PREPARATION OF SUITABLE PARTICLES BY ROTATIONAL CASTING IS ALSO DISCLOSED.

June 22, 1971 s F, RADTKE ETAL 3,586,747

METHOD OF OBTAINING STRIPS AND SHEETS OF ZINC AND ZINC ALLOYS Filed April 9, 1969 4 SheetsSheet 1 INVILN'IURS SCHRADE F. RADTKE a By GIOVANNI SCACCIATI their ATTORNEYS June 22, 1971 s. RADTKE ETAL 3,586,747

METHOD OF OBTAINING STRIPS AND SHEETS OF ZINC AND ZINC ALLOYS Filed April 9, 1969 4 Sheets-Sheet 2 INVIL'N'IURS SCHRADE F. RADTKE 8 By GIOVANNI SCACCIAT] their ATTORNEYS June 22,197

Fil ed April 9, 1969 1 s. F. RADTKE ETAL METHOD OF OBTAINING STRIPS AND SHEETS OF 'GREEN STRIPS Kg/mm ROLLING TEMPERATURE GREEN S RIPS Kg/mm 20 LUZ 2 I y/ 2: E a 8 ROLLING TEMPERATURE REROLL STRIP Z E Q s NUMBER OF REROLLINGS ZINC AND ZINC ALLOYS 4 Sheets-Sheet 5 FIG. 7

INVENTORS SCHRADE F. RADTKE 8: BY GIOVANNI SCACCIATI g/Lwmau 4 w/A /)J773Au p a "@(1 their ATTORNEYS June 22, 1971 S. F. RADTKE ETAL METHOD OF OBTAINING STRIPS AND SHEETS OF ZINC AND ZINC ALLOYS Filed April 9, 1969 ELONGATION (LONGITUDINAL) b co GREEN STRIPS ROLLING TEMPERATURE (LONGITUDINAL) NUMBER OF REVERSE BENDS NUMBER OF REVERSE BENDS LONGITUDINAL TRANSVERSE ROLLING TEMPERATURE REROLLED STRIPS NUMBER OF REROLLINGS 4 Sheets-Sheet 4 GREEN STRIPS ROLLING TEMPERATURE FIG. /0

FIG. /2

INVIiNlYi/(S SCHRADE F. RADTKE 8- y GIOVANNI SCACCIATI their ATTORNEYS United States Patent Office US. Cl. 264-6 Int. Cl. B22f 3/18 4 Claims ABSTRACT OF THE DISCLOSURE A method for preparing sheets or strips of zinc or alloys of zinc by direct rolling of metal particles is disclosed. The size of the particles is preferably above a certain minimum, that is, larger than about 0.2 mm. in the smallest dimension, and the temperature and pressure of the rolling are also above certain minimums, that is, sufiicient to effect in a single rolling pass a percent reduction of the metal or alloy of at least about 80% from the original thickness of the compacted particles. A method for the preparation of suitable particles by rotational casting is also disclosed.

BACKGROUND OF THE INVENTION This is a continuation-in-part application of application Ser. No. 647,698, filed June 21, 1967, and now abandoned.

The present invention relates to the production of continuous lengths of solid metal from east particles or granules of zinc or zinc alloys by direct rolling of the particles.

The formation of solid metal bodies from metal powders by pressing and sintering is known. The presence of thin oxide films on the surface of metal particles, however, presents problems in such a process because the oxide film may reduce or prevent bonding of the particles to form a coherent body of the metal. In the past this problem has been dealt with by using special sintering furnaces and nox-oxidizing or reducing atmospheres to prevent the formation of oxide films or to remove such films from the particles. It has also been known to form cast billets of zinc metal or zinc alloys and reduce them to the form of sheets or strips by rolling either longitudinally, or longitudinally followed by transverse rolling. Such procedure, however, requires large capital expenditures for plant facilities and equipment and necessitates a large labor input to cast and handle slabs, or billets, large enough to be rolled into sheets of commercially usable thicknesses and lengths.

Moreover, conventional procedure is relatively inflexible in regard to the types and quality of the zinc alloy strips produced. In rolling zinc strips from cast billets, for example, difficulties are encountered in obtaining a homogeneous distribution of alloy additives because of the tendency of certain additives to separate from molten zinc and precipitate out upon solidification of the billets. Certain zinc alloys of suitable quality cannot, therefore, be produced by conventional techniques, with the result that a low quality product is used or an otherwise unsatisfactory substitute is found for the intended use.

A further problem encountered with known procedure, is that it has been difiicult to obtain zinc sheet having isotropic properties, especially in the longitudinal and transverse directions. This disparity is caused in conventional rolling procedure by subjecting the cast billets to a plurality of rerollings by which the thickness of the sheet is gradually reduced. As a result of the multiple reroll- Patented June 22., 1971 ings, the zinc crystals are elongated primarily in the rollmg direction and only to a much lesser extent in the transverse directon.

SUMMARY OF THE "INVENTION Accordingly, it is an object of this invention to provide a method for the formation of continuous lengths of zinc or zinc alloys by which substantial reductions in capital investment and labor requirements over those associated with conventional rolling procedure can be realized.

Another object of the invention is to provide a method for the preparation of coherent bodies of zinc or zinc alloys by consolidation of discrete particles, which does not require the use of special reducing or non-oxidizing atmospheres.

Another object is to provide a method for producing arm and zinc alloy strips or sheets having improved lsotropy of physical characteristics.

Another object is to avoid the necessity of casting and handling large ingots or billets which are required to produce long lengths by conventional rolling techniques.

A still further object of the invention is to provide a method in which continuous strips or sheets of zinc or zinc alloy metal can be obtained from east particles in a single rolling operation.

A still further object of the invention is to provide a method for the formation of zinc and zinc alloy sheets or strips containing alloy additives not susceptible of being cast, or which cannot be cast in a practical manner, in zinc alloy billets.

These and other related objects are achieved by a process in which zinc or zinc alloy particles, formed preferably by a rotational casting operation, are preheated and then compressed between rolls, with the temperature of the particles and the pressure applied by the rolls being controlled so as to effect in a single pass through the rolls, a reduction in the thickness of the compacted particles of at least of the original thickness of the compacted particles. The temperature is selected to be high enough to allow the 80% reduction to be achieved without the necessity of applying too high pressures by the rolls. Advantageously, the preheat temperature of the particles should be close to the melting point of the metal or alloy, for example, within about C., but not so close to the melting point that melting of the particles occurs either prior to or during the rolling operation. The temperature of the particles at the nip of the rolls, therefore, is selected so that the rolling does not raise the temperature above the melting point of the metal or alloy.

In accordance with the invention, zinc or zinc alloy particles are obtainable at very low cost and with significantly less labor than is required to cast the large billets needed to obtain sheets or strips in usable lengths by conventional rolling techniques. A further reduction in labor requirements is realized by the present invention in that sheets of commercial thickness can be obtained in a single rolling operation, and need be followed only by finishing skin pass treatment to adapt the rolled sheet to particular applications. Moreover, the equipment needed to cast the particles and to carry out the rolling operation is substantially less expensive and complex than the equipment required in a conventional plant and, accordingly, significant reductions in capital investments are also realized by this invention.

BRIEF DESCRIPTION OF THE DRAWINGS Referring to the drawings, FIG. 1 is a schematic drawing illustrating the various zones of metal particles as they pass into and through the nip between the compacting rolls;

FIG. 2 is a planar representation of the crystal texture, as determined by X-ray diffraction, of zinc strip produced from billets by conventional rolling techniques;

FIG. 3 is a similar representation of the crystal texture of zinc strip produced from particulate zinc in accordance with the present invention;

FIG. 4 is a schematic drawing of the apparatus employed for casting and subsequently compressing the metal particles;

FIG. 5 is a sketch of the compacting rolls shown in somewhat greater detail;

PG. 6 is a graph showing the relationship between longitudinal tensile strength and rolling temperature for a number of strips rolled at selected pressures;

FIG. 7 is a similar graph showing the relationship between transverse tensile strength and temperature;

FIG. 8 is a graph showing the relationship between longitudinal and transverse tensile strength, and number of rerollings;

FIG. 9 is a graph showing the relationship between longitudinal elongation and rolling temperature;

FIG. 10 is a graph showing the relationship between the number of reverse bends (longitudinal) vs. rolling temperature;

FIG. 11 is a similar graph showing the relationship between the number of reverse bends (transverse) vs. rolling temperature; and

FIG. 12 is a graph showing the relationship between the number of reverse bends (longitudinal and transverse) vs. number of rerollings.

DESCRIPTION OF A REPRESENTATIVE EMBODIMENT Preferred materials for use in the process include essentially pure zinc, such as electrolytic zinc, and wrought zinc alloys which normally contain not more than 3% of alloying metals. These wrought zinc alloys are well known as such. The following table illustrates the type of materials that can be used and their compositions.

It will be observed that the amounts of alloying metals for these wrought zinc alloys are quite low and, accordingly, the melting point will be almost the same as that for electrolytic zinc, namely, about 4l9.5 C. However, the pressure ranges required for rolling or compression of the zinc alloys (Items (2) and (3) in Table I plus those described below) may be up to higher than for electrolytic zinc.

Besides electrolytic zinc and the wrought zinc alloys described above, other alloys which can be used include zinc-titanium chromium, and zinc-aluminum copper wrought alloys. These alloys also contain less than 3% alloying metal. It is possible, however, to utilize zinc alloys containing substantially larger amounts of alloying metal up to 49% thereof, the major component zinc being present at a minimum of 51%. For example, alloys of zinc and aluminum containing to aluminum may be used.

The process of the present invention also makes possible the production of new alloys of zinc which could not be produced by conventional techniques because of the insolubility of certain alloy additives in molten zinc or because of the tendency of the additives to precipitate out upon solidification of the billet. This provides in creased flexibility in the fabrication of new products to meet the requirements of a given application, and yields economic and other advantages over the products of conventional milling techniques.

Further advantages in the production of alloys result because of the relative ease with which the separate additives can be alloyed with the particulate zinc. Specifically, the elements can be mixed with zinc while either or both are in the molten state, and the molten alloy can then be cast centrifugally to produce alloy particles, or the additives and the zinc can be cast separately and later admixed as discrete particles prior to being passed through the rolls. In this way, substantial savings are realized in that the alloy additives and the zinc can be cast asparticles with little labor and capital investment. It will be apparent, of course, that both metallic and nonmetallic particles can readily be added to the particulate zinc in order to produce a rolled strip of desired properties.

As noted above, it is important for the purpose of the present invention that the rolling pressure and temperature be selected so as to produce at least a minimum percent reduction in a single pass through the rolls. With this object in mind, the temperature preferably is maintained within 100 C. of the melting point of the zinc or zinc alloy in order that advantage can be .taken of the decreased strength of the particles as the melting point is approached so that the desired percent reduction can be efiected at reduced roll pressures. The minimum percent reduction in a single pass is approximately with improved results being obtainable at higher reduction percentages. There is no express upper limit on the amount of reduction but it is governed mainly by practical considerations such as the amount of pressure which can practically or feasibly be applied to the particles by the rolls.

It will be appreciated that the rolling temperature need not be within C. of the melting point, but rolling temperatures selected within this range are advantageous in that the improved plasticity exhibited by the metal when heated to near its melting point can be exploited to the maximum and they allow the percent reduction to be carried out at reduced rolling pressures. That lower rolling temperatures can be used, is important from a practical view point because it is difficult to roll the particles into strip at temperatures near the melting point without causing some melting of the metal particles. By preheating the metal particles to a temperature suitably lower than the melting point, the particles can be rolled without being raised to a temperature higher than the melting point by the consequent temperature increase resulting from the rolling operation. It is preferred, therefore, to maintain as high a temperature as feasible so long as the danger of melting of the particles is avoided, either at the preheat stage or at the rolling stage.

Rolling at temperatures within 100 C. of the melting point is possible with the method of this invention, notwithstanding that they fall within the hot shortness (in this case) range of zinc and zinc alloy metals. The term hot shortness is used to designate the phenomenon occuring when an attempt is made to roll a billet of zinc or zinc-base alloy while the metal is at a temperature within the range of from about 300 C. to 419 C. Typically, fracture of a conventional billet occurs due to the release of bending stresses between crystal grains, with the result that the rolled sheets or strips contain cracks and other discontinuities.

It is though that the reason why it is possible to roll consolidate the particles in the hot shortness range is because the small, fine-grained crystals in the particles are random oriented and thus provide more freedom for twinning and slip. By comparison, the large, columnar grains of conventionally cast billets are arranged in a parallel array in the direction of solidification, and tend to hot tear along the grain boundaries when subjected to strain at high temperatures.

The manner of measuring the percent reduction can be explained and understood by referring to FIG. 1 which is a schematic illustration of two rolls in cross-section and particles of metal being fed between the rolls. A horizontal mill, that is, one in which the two rolls are mounted side by side in a horizontal plane is illustrated. The particles designated generally by reference numeral 10 are introduced above the nip as shown. The zone of the loose particles is indicated by a. Approximately at the plane designated at 11 the particles begin to bond to each other due to the pressure applied by the rolls. In the zone b, therefore, designated at 12, the bonding of the particles to each other is taking place but there is still "a considerable amount of voids.

At the plane designated by 15 the particles are substantially completely bonded to each other and the percentage of voids is very low, that is, about This is the reference point from which the percent reduction is measured. As is explained more fully hereinafter, the point where substantially no more than 5% 'voids exists is taken as a reference because it allows ready, yet precise, determination of the point of transition from a condition where voids exist to a condition of substantially no voids. Thus, in the zone 0 bonding and distortion of the particles to each other and also the reduction of the sheet is taking place, which continues until the point of minimum spacing 17 is reached. The percent reduction, therefore, is equal to the thickness at plane 15 minus the sheet thickness at plane 17 times 100, divided by the thickness at plane 15, and preferably is 80% or higher. It is an important feature of this invention that this percent reduction is. accomplished in the first pass through the rolls. The fully compressed sheet is designated by d.

The particle size of the metal is also significant, and preferably is about 0.22 mm., or 200 am, in the smallest dimension. The principal reason for this is that smaller particles, due to their larger surface areas per unit weight may have too high a content of oxide present as a film on the surfaces of the particles. With respect to the maximum size of particles that can be used, the type of equipment again has some significance. In a vertical mill with rolls having a diameter of about 25 cm. there may be some ditficulty in the feeding of particles larger than about 1 mm. because of the angle of the nip between the rolls, that is, particles larger than about 1 mm. are not readily forced into the nip between the rolls because of problems of insufficient friction between the particles and the roll surface. With a horizontal mill, or with a vertical mill having larger diameter rolls, however, these factors are not as significant and, accordingly, particles as large as 2 mm. and above may be used.

The roll pressure is a function of many factors, such as temperature of the particles as compressed, percent reduction, roll speed, composition of the metal, roll diameter, etc. Pressure may vary, for instance, from as low as about 30 kg. per sq. mm. to 70 kg. per sq. mm. or above. For a 60 kg. per sq. mm. pressure, in the case of a vertical mill having 25 cm. diameter rolls, the force on the rolls would be about tons per linear cm., that is, a 1000 ton apparatus would be required for obtaining a commercial strip '1 meter wide.

It will be understood, of course, that the term percentage of voids refers to the percentage of the total cross section of the bonded particles that is not occupied by the particles, that is, that percentage of the cross sec tion constituted by voids. The percentage of voids, and concomitantly, the extent of bonding together of adjacent particles, may be determined in any suitable manner. A preferred manner is by making several runs through the rolls, stopping the rolls during each run, and taking samples of the full length of the consolidated and partly consolidated material between the rolls. Thereafter, the samples are sectioned, and the cross sections of the samples examined with a high-powered magnifier to determine the point on the respective cross sections at which substantially no more than 5% voids exist, or, in other words, the location of the plane of the drawings from which measurement of the percent reduction is to be made. This procedure allows accurate determination, by visual examination of a section of the material, of the point along the cross section where gross porosity disappears. It therefore greatly facilitates the process of locating the point of transition from a condition of low percent voids to a condition of zero porosity and, for given particulate material and roll conditions, introduces no appreciable variation in the location of the plane 15 and hence in the thickness against which the percent reduction is to be measured.

By following the procedure of the present invention, it is possible to obtain in a single rolling operation a strip or sheet of zinc or zinc alloy which has very good properties, and in which improved isotropic characteristics are obtained. In rolled sheet or strip in general, and in zinc rolled sheet or strip in particular, small crystal grains are given preferred orientations in a manner determined by the nature of the metal and rolling and other working conditions. It is possible, therefore, to determine the crystal texture of the rolled product by X-ray diffraction techniques, and the determinations thus made can be conveniently represented on a plane by sterographic projection. In FIGS. 2 and 3, such representations have been made for zinc strip rolled from billets in the conventional manner and for zinc strip rolled from particulate zinc in accordance with the present invention, respectively.

A comparison of FIGS. 2 and 3 clearly shows that better isotropy is obtained in the sheet rolled from particulate zinc. It is to be noted in connection with FIGS. 2 and 3 that the axes labeled R.D. are parallel to the rolling directions and the axes labeled CD. are transverse to the rolling direction of the sheet. In the case of conventionally rolled sheet (FIG. 2), the hexagonal axes of the crystal grains are assembled in the neighborhood of two poles that are inclined at approximately 22 to the perpendicular to the plane of the sheet. On the other hand, in the case of the rolled particulate sheet, the hexagonal axes of the crystal grains are assembled in the neighborhood of a single pole placed near the perpendicular to the plane of the sheet (FIG. 3). Because the directions perpendicular to the hexagonal axes have equivalent characteristics, the rolled particulate sheet demonstrably shows a better isotropy than does the conventional sheet.

It is a surprising and important feature of the invention that the improved isotropy is best obtained only when the percent reduction in the first pass through the rolls is or greater. That is, by effecting this high percent reduction in a single pass, the particles are elongated simultaneously in both the rolling and transverse directions, and the fine crystal grains of the particles are thereby uniformly distributed wtihin the sheet, as indicated by the distribution pattern of FIG. 3. Conversely, if the same percent reductiOn was accomplished by multiple passes, as is the case where strip is produced from billets, the crystal grains are not uniformly distributed but are assembled generally in the rolling direction. As previously noted, random crystal orientation of the rolled particulate sheet is also of special advantage in that it allows rolling to be done within the hot shortness range, with full exploitation of the increased plasticity of the metal existing within this range, while avoiding the hot shortness phenomenon that occurs during conventional rolling at these temperatures.

Still another interesting and surprising feature of the present invention is that it is possible to obtain in a single rolling operation properties of the zinc strip or sheet which cannot be obtained by the combination of first rolling at lower pressures or temperatures (with substantially lower percent reductions) followed by subsequent rollings either longitudinally or transversely eventually to obtain a product having the same thickness as that obtained in the single rolling with the high (80% or more) reduction. Sheet or strip consolidated in accordance with the invention contains small amounts of oxide in dispersed form, derived from the oxide film which coats the individual particles. It is known that the presence of dispersed oxide hinders the recrystallization of the rolled particulate sheet which, as noted above, has finer grains than does rolled zinc strip produced by conventional methods. As fine crystal grains, other conditions being equal, reduce creep strength, it should be expected that rolled products obtained according to the invention would have a lower creep strength than the conventionally produced products. Surprisingly, it has been found that the rolled particulate products instead have a higher creep strength than the conventional products. It is believed that this results from the presence of the dispersed oxide which, although hindering recrystallization of the zinc, acts as an obstacle to slip at the intercrystalline boundaries, and as slippage between crystals is the main cause of creep, the dispersed oxide therefore is effective to restrain creep.

Metal particles of suitable size and shape for rolling can be cast by several techniques. For example, particles can be cast by passing molten metal through a perforated plate into a reservoir of air, water, oil, or some other quenching liquid.

The preferred method for producing metal granules is by centrifugal casting. This method allows good control of both the size and the shape of the granules through control of the diameter of the holes through which the metal is cast and by control of the speed of the casting apparatus which creates the centrifugal forces. In centrifugal casting, molten metal is fed into a rotating container which is maintained at a temperature above the melting point of the metal. The container is provided with suitable holes through which the metal will pass. The size of the holes depends, of course, on the size and shape of the particles to be made. Suitable holes range from about 0.1 to about 3 mm. in diameter. The metal is cast from the container by centrifugal force and divided into droplets which solidify during flight. The solidified droplets are then collected in a suitable collecting chamber. Any type of rotating device or container can be used in centrifugal casting, for example, flat disks, conical disks, grooved disks, or flanged conical containers equipped with a cover in a manner such that the metal enters through the center opening and is discharged through a slot between the cover and the edge of the container.

The droplets may be caused to solidify by allowing them to fall into a container filled with water or other liquid quenching agent. An air quenching bath, however, is preferred. Any oxidation of the metal particles which may take place in the centrifugal casting apparatus can be minimized by employing a controlled atmosphere such as nitrogen, which can easily be designed into the apparatus. Should high creep strength strip be desired, it is desirable, as shown above, that some oxidation of the particles occur.

The use of centrifugal casting provides granules having a more or less lengthened or prolate shape and a size preferably ranging from about 0.2 to about 2 mm. in their smaller dimension. Centrifugally cast particles are substantially different from powders obtained by atomizing, grinding, or other conventional methods. Centrifugally cast granules have substantially larger size than metal particles produced by other methods. The increase in size is desirable, since it decreases the surface to volume ratio, thereby reducing the amount of oxide film to levels which do not prevent or diminish consolidation of the metal into a solid body or strip. Such granules are of a substantially uniform size and form a free-flowing mass, characteristics which make them especially suitable for subsequent rolling.

FIG. 4 of the drawings shows a schematic arrangement of equipment which may be used for the production of particulate material from molten metal and subsequent conversion to wrought sheets. Ingots of metal are fed into a melting furnace 25 where the metal is first melted, then raised up to casting temperature. The higher temperature is required to obtain optimum results and to prevent excessive cooling in the spinning cup and subsequent plugging of the cup orifices. From the furnace the molten metal is conducted by means of a transfer pipe 26 to the spinning cup 27 in the centrifugal casting apparatus 30. The spinning cup is rotated by a variable speed motor 31. The spinning cup, which is easily fabricated from steel, cast iron, graphite, or other refractory materials essentially inert to molten zinc, is preheated only in the beginning of the operation. Subsequent heat requirements are supplied by the molten metal itself as it flows into the cup.

The cup, as stated, is caused to rotated by the motor 31 at a speed which can range from about 100 to 5,000 rpm. The molten metal is carried through the perforations in the cup by centrifugal force over a distance which may vary in accordance with the type of device and rotational speed. The metal is thus divided into droplets which solidify during flight before they reach the walls 32 of the centrifugal casting apparatus. Resulting metal particles are cooled at 35 after passing downwardly over the inclined conical surfaces 36 of the apparatus 30. Vibrators 37 are preferably mounted on the under surfaces of the reverse conical plate 40 and the inclined conical surface 36 to facilitate the passage of the particles over such surfaces.

By use of this apparatus, zinc and zinc alloy particles can be obtained at very low cost. The production of the particles obviously requires less labor than does the casting of large billets, as is necessary to obtain sheet or strip in sufficient lengths in conventional rolling mills. Also, substantially less capital need be invested in equipment and plant facilities.

Although not shown in the drawings, it will be understood that suitable apparatus for admixing alloy additives in particulate form with the particulate zinc can be positioned between the casting apparatus 30 and the rolls for casting and mixing together the separate particles in the desired proportions. By this method, a homogeneous distribution of alloy additives is accomplished and the segregation which sometimes occurs during the solidification of cast billets is avoided. A further advantage associated with adding the alloy elements as discrete particles, therefore, is that new products previously not susceptible of manufacture from zinc billets can be produced. Where the additives are soluble in molten zinc, they can, of course, be introduced into melting furnace 25, and the zinc alloy then cast as particles.

A conveyor system 41 is provided to elevate the cast particles and deposit them into the preheater 42. The latter may be heated by any suitable means such as electricity or gas. The particles are deposited onto the upper surface of the conveyor 45 which is mounted therein and are preheated during the course of their travel through the preheater. The particles leaving the preheater 4-2 are deposited into the metering feeder 50 which also has its conveyor system 51. The rolls 52 of the metering feeder 50 are provided with a suitable controlled speed motor in order to control the rate of discharge of the cast metal particles from the metering feeder. The particles from the metering feeder 50 are then deposited into the hopper 55 which is provided with vibrators 56.

The particles are deposited from the hopper 55 into the nip between compacting rolls 70, which are described in greater detail below. The sheet of metal issuing from the compacting rolls 70 passes between the guide rolls 91 and thereafter passes through the slitter 92 where rough edges are removed from the sheet, after which the trimmed sheet is coiled onto the coiler 95.

Referring then to FIG. 5, which is a view of the compacting rollers in greater detail, the two rolls are shown as 70a and 70b mounted on horizontal axes disposed in a horizontal plane. It is important for the purpose of the present invention to supply a suitable lubricant to prevent sticking of the strip 90 to the rolls. Liquid or solid lubricants such as graphite can be employed. It will be appreciated that the quality and quantity of the lubricant is controlled in order to obtain adequate lubrication and also to preserve the quality of the rolled strip.

Moreover, during rolling additional heat is generated by the energy dissipated by the mechanical deformation of the metal. For this reason the rolls may be cooled by known means. For example, hollow cylinders containing circulating cooling fluid may be used. Alternately, the rolls can be cooled by the use of water sprays which play onto the surface of the roll in such a manner and in such a quantity as to be heated and evaporate before that portion of the roll surface comes into contact with the granules. In accordance with still another procedure lubricant may be applied to the surface of the rolls and simultaneously accomplish both cooling and lubrication.

Pursuant to the foregoing, there is shown in FIG. lubricating means 71 which may be of any conventional type adapted to supply lubricant to the outer surfaces of the rolls. If desired and/or necessary there also may be supplied additional or alternative cooling means 72 which is also constructed in accordance with known technology adapted for removing heat from the rolls 70a and 70b.

For the purpose of preheating the rolls to the desired operating temperature, there is preferably provided preheating means 75 which may suitably be a gas burner as indicated schematically in FIG. 5. Once the apparatus has reached steady operation, heating of the rolls 70 generally will be unnecessary because the rolls will receive sulficient heat from the preheated particles themselves and from the additional heat generated in th particles by the mechanical working to which they are subjected.

The final thickness of the roll sheet is, of course, variable so that zinc or zinc alloy sheet suitable for different uses can be produced. Generally, however, the final thickness would be on the order to 2 mm. Sheet of this thickness can of course, be subjected to further rolling, as, for example, in a finishing mill, to perfect the appearance of the surface of metal, or to reduce the gauge of the sheet, or both.

EXAMPLE The granules used for preparing a zinc strip were in the size range from 0.2 to 1 mm. and had the following chemical composition.

Percent Pb 0.0015 Fe 0.0030

Cd 0.0010 Cu 0.0008

Zn Balance zero porosity. Micrographic examination of the material also proved that there is essentially no porosity. Besides the zinc strips which were produced by a direct rolling in a single rolling operation, still others were subjected to a number of rerolling steps. In this group the first rolling operation took place at C. and kg./mm. pressure to a thickness of about 1 mm. These were rerolled cold to a final gauge of 0.4 mm. This reduction was obtained either in a single rerolling or in a number of rerollings each with a reduction of 0.05 mm.

Still other strips were obtained, also from electrolytic zinc, by rolling a billet of the metal in accordance with conventional procedures. One such material was obtained having a thickness of 1 mm. and another having a thickness of 0.4 mm. The following measurements were made on all of the materials:

(a) Tensile strength to fracture in longitudinal and transverse directions;

(b) Percent elongation to fracture in longitudinal direction;

(c) Number of bends in longitudinal and transverse directions;

(d) Sharp corner pliability in longitudinal and transverse directions; and

(e) Viscous creep under constant load in longitudinal direction.

TENSILE STRENGTH TESTS The tensile strength tests in the longitudinal direction were conducted with small rectangular test pieces of 20 mm. width having working lengths of 100 mm. Breaking elongation was also determined. For the test pieces for measurement of transverse properties similar, although shorter, test specimens were used.

PLIABILITY TESTS The pliability tests were made on the single rolled strips and rerolled strips by determining the number of reverse bends at 90 needed to produce breaking, over a curvature radius of 10 mm., and while subjecting the test piece to a tension of 0.5 kg. per sq. mm.

Other specimens were also evaluated with respect to their behavior upon sharp angle bending. In this series the sample was folded back tightly in a vise and the appearance of the bent portion was evaluated in accordance with the following criteria:

E-'Excellent, the angle was smooth and intact;

G-Good, the angle was slightly creased without cracks; FFair, the angle was sharply creased; BBad, the angle was cracked; and VBVery bad, there were fractures over the entire length of the angle.

CREEP The study of the creep was made by applying directly to the test piece a 5 kg./mm. constant load and observing the elongation time of the test piece until breakage. The index for evaluation of creep resistance chosen was the ratio between the duration (in hrs.) and the percentage in elongation, i.e. the reciprocal value of the elongation rate.

TABLE II Percent reduction Thickin the Tensile Tensile Elon- Reverse Reverse Creep, ness, compaction strength, strength, gation, bends. bends, Zero T Zero T hr./A Specimen mm pass kgJmmfi kgJmmfl percent N N bend a bend b percent Line From billetsuugsheet 1 15 21 25 90 20 E E 2 a --do 0.4 15 20 26 200 80 E E 3 1; Single rolled strip, 20 0., 25 kgJmml- 1 8 2 10 VB VB 2 0 Single rolled stn'p, From partlcles 400 0., 60 kg./mm.'- 0. 4 80 15 18 8 150 E G 6 d Single rolled strip, 20 0., 40 kg./mm. 0. 70 12 9 4 40 10 B VB 2 e Rerolled 1 pass 0. 4 12 13 12 700 40 B VB 2 f Rerolled 7 passes 0.4 12 13 10 200 20 B VB 2 g Longitudinal direction. Transverse direction.

Table II includes (in lines a and b), data obtained from measurements of the characteristics of ordinary rolled electrolytic zinc strips obtained by rolling a plate to two different final thicknesses. Table II also contains comparable data on the characteristics measured for strips obtained by direct rolling of granules (lines to g). The values obtained for the single rolled strips at 25 kg./mm. and 20 C. and at 60 kg./mm. and 400 C. appear in lines 0 and d, respectively. The data appearing in lines and g were obtained from a strip which was first rolled at 40 kg./mm. and 20 C. to produce a strip 0.75 mm. thick (line e) and subsequently rolled in either one pass (line 7) or seven passes (line g) to a final thickness of 0.4 mm. The percent reduction in the compaction pass is listed in Table II for each of the single rolled strips, otherwise referred to as green strips.

Comparing lines a and c which refer respectively to the properties of a conventional strip and a single rolled strip of equal thickness, but rolled at a relatively low pressure and temperature with only 60% reduction, it is evident that the properties of the former are definitely superior to those of the latter. This is explained on the basis that the sheet rolled from granules was produced at too low a pressure and temperature, and thus at too low a percent reduction. Making a comparison, however, between the conventional rolled strip and the single rolled strip from granules obtained at a higher pressure and temperature with a full 80% reduction (line d) it appears that the properties of the tensile strength, number of reverse bends and pliabili-ty, are comparable. The strip obtained directly from the granules also had a lower elongation and twice the resistance to creep. The properties of the rerolled strip obtained from the single rolled strip of line e, indicated in lines 1 and g, are inferior with regard to pliability and tensile strength as compared with the single rolled strip produced at 80% reduction (line d). Only the number of reverse bends in the longitudinal direction is much higher for the multiple rolled strip but this is considered to be relatively unimportant as compared with the substantial superiority of the single rolled strip (line d) with regard to transverse bending. From these data it is apparent that rerolling, while improving the properties of strip obtained under conditions less than optimum, does not compare favorably to the properties of a strip obtained by rolling the particles at high pressure and temperature with a high percent reduction.

In FIGS. 6 to 12 the results of the tests carried out on the various strips as described above are presented graphically and in grater detail.

In FIGS. 6 and 7 there is presented for the various single roll strips, or green strips, the tensile strength in the direction of rolling (longitudinally) and in the transverse direction, as a function of the rolling temperature of the granules. The various straight lines in the figures represent the values of tensile strength for the various rolling pressures indicated as follows:

A=25 kg./mm. B=30 kg./mm. C=40 kg./mm. D=60 kg./mm.

It will be appreciated by those skilled in the art, that at the higher temperatures and pressures, a higher percent reduction is effected, and the properties of the strips are seen to improve substantially with increase in percent reduction.

It will be observed that when the pressures and the temperature of rolling are increased, there is an increase in the tensile strength both in the longitudinal direction and particularly in the transverse direction. The value of the tensile strength in the longitudinal direction of the electrolytic zinc rolled from a billet (designated by the triangle A on tht ordinate) is practically the same as that of the single rolled sheet from granules for the material rolled at higher pressure 60 kg. mm. and higher temperature (400 C.). In the transverse direction the tensile strength of the conventional rolled strips (the solid triangle A on the ordinate) is only slightly higher (FIG. 7). Transverse tensile strength data were not obtainable for rolling pressures of 25 and 30 kg./mm. because the strips produced were too narrow for test specimens of the required size to be taken.

In the diagram of FIG. 8, the tensile strength of rerolled strip in the longitudinal direction (continuous line) and in a transverse direction (broken line) is plotted as a function of the number of rerolling passes. This again refers to the rerolling of material initially rolled from granules at C. and 40 kg./mm. The rerolling process as applied to the single rolled product obtained from granules at 20 C. and 40 kg/mm. increases substantially the tensile strength in the transverse direction but this transverse tensile value is still substantially lower than the transverse tensile srength of the single rolled sheet from granules at 60 kg./mm. and 400 C. On the axis of the ordinate the circle O and the solid circle indicate the values of the tensile strength in the longitudinal and transverse directions, respectively, for the single rolled strips before rerolling. The triangle A and the solid triangle A indicate the longitudinal and transverse tensile strength, respectively, of the conventionally rolled strip. It is apparent that the increase in tensile strength is more marked in (the transverse direction and is independent of the number of rerolling passes, in the case of sheet rolled from granules.

It is apparent also that an increase in temperature and pressure of rolling, and thereby in percent reduction, results in an increase in the percent elongation to rupture of the strips (FIG. 9). The elongation value for the conventional rolled strips (designated by the triangle A on the ordinate) is much higher than that for the single rolled strip from granules.

In the diagrams of FIGS. 10 and 11 there is presented the number of reverse bends for the various strips in the longitudinal direction (FIG. 10) and in the transverse direction (FIG. 11), as a function of the rolling temperature. From these data it appears that the number of reverse bends in the two directions increases with an increase in rolling temperature and pressure. In the longitudinal direction the number of bends of the strips obtained at higher rolling temperature and pressure reaches 150, whereas in the transverse direction it reaches 70 bends. The number of bends obtained with conventional rolled strips in both directions is only slightly higher (as designated by the open triangle A and solid triangle A on the ordinates of FIGS. 10 and 11, respectively).

Again, the transverse direction (FIG. 11) data are not available for green strips rolled at kg./mm. and kg./mm. because of the narrowness of the strips.

In FIG. 12 there is presented data on the number of reverse bends in the longitudinal and transverse directions of material rerolled, that is, initially rolled at kg./mm. and 20 C. (designated by the open O and solid 6 circles on the ordinate, respectively). With rerolling, the number of reverse bends in the longitudinal direction increases substantially and passes the value obtained for the conventional rolled strips (designated by the open A triangle on the ordinate), especially if reduction is obtained with a low number of passes. The number of reverse bends, however, increases only slightly in the transverse direction after rerolling.

The foregoing clearly demonstrates that the properties of the strips obtained directly from granules are related to the conditions under which the strip is obtained, that is, principally the percent reduction, as represented in terms of rolling temperatures and pressures. The strips obtained at room temperature and low rolling pressure (25 kg./mm. show incomplete densification. Micrographic examination shows that the granules are not markedly deformed and do not adhere completely to each other. On the other hand, at higher pressures kg./

mm?) and higher temperatures (400 C.), deformation of the granules is much more pronounced and adherence is complete. The barriers created by the oxide which covered the surface of the granules appear to be closer together and arranged parallel to each other. This is verified by electron microscopic observation. With the more deformed granules, that is, those subjected to a higherpercent reduction, there is obtained a better distribution of the oxide in the metal matrix and a general improvement in all of the physical characteristics of the rolled strips.

In the strips originally rolled at low temperature and pressure and subsequently rerolled, the oxide barriers are closer together and there is a more homogeneous distribution of the oxide phase. This, however, was not capable of producing acceptable transverse properties in the finished sheet.

We claim:

1. A method for producing a fully densified metal strip or sheet having improved isotropic characteristics from prolate shaped particles of zinc or zinc alloy having a minimum dimension of at least 0.2 mm., comprising:

introducing the particles of zinc or zinc alloy between spaced compacting rolls, and

rolling the particles at a temperature within the range of from 300 C. to 400 C. and at a pressure sufficient to produce, in a single compaction pass through the rolls, at least an 80% reduction in the thickness of the strip or sheet, the percentage reduction in thickness being defined by (a) the intermediate thickness of the compressed 14 particles at a point during the passage thereof through the rolls where the particles are compacted to an extent that substantially no more than 5% voids exists among the particles minus (b) the thickness of the strip emerging from the rolls, (0) all divided by the intermediate thickness and multiplied by 100.

2. The method according to claim 1 wherein the particles are produced by rotational casting.

3. The method according to claim 1 wherein the compression is eflected between horizontal rolls mounted with their axes in a horizontal plane and the particles are introduced downwardly into the nip between the rolls.

4. The method according to claim 1 further comprising admixing particles of zinc and particles of alloy additives and compressing the admixed particles to produce a sheet or strip of zinc alloy.

References Cited UNITED STATES PATENTS 3,246,982 4/1966 Moritz et a1. 2648 3,334,408 8/1967 Ayers 264111 3,329,746 7/1967 Joyce et al. 264-8 ROBERT F. WHITE, Primary Examiner I. R. HALL, Assistant Examiner US. Cl. X.R. 264-111, 8

PO-IDSO Patent No.

Dated June 22 lQ7l l v ntm-(s) Schrade F. Radtke and Giovanni Scacciati and that said Letters Patent are hereby corrected as It is certified that error appears in the above-identified patent Column Column Column Column Column Column Column Column Column shown below:

line &4 line 43 line 62 line 30 "nox" should be --non--; "T" should be --Ii--,' "though" should be -thought-; "0.22 should be --O.2--,' line 48 "wtihin" should be --within--; line 11 "rotated" should be --rotate--,'

[in heading of Table II) "Elongation, percent" should be "Elongation, a percent---; 11, line 48 "grater" should be --greater--; 11, line 73 "tht" should bethe Signed and sealed this 21st day of March 1972.

(SEAL) Attestz EDWARD M.PLETCHER",JR. Attesting Officer ROBERT GOTTSCHALK Commissioner of Patents 

